Novel photochromic coatings were designed having a remarkable long-term stability of both the coloured state and colourless states in the dark (bistability). The coatings consist of a dispersion of a commercial spirooxazine in a functionalized sol–gel hybrid matrix, acquiring a deep purple coloration upon irradiation with UV-light. In order to stabilize the coloured form of the dye, a zinc salt, capable of forming a chelate with the photochromic molecule when irradiated, was incorporated into the matrix, resulting in a drastic reduction of the thermal bleaching kinetics. The coloured coatings can only be reverted to their original colourless state by irradiation with visible light. This recording–erasing process is reversible and can, therefore, be repeated by alternating irradiation with UV and visible light. Adding the bistable behaviour to the specific properties of reversible photochromic systems makes a new range of applications in the field of optics and light-controlling media available.
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